Circuitry which operates at high frequencies generates signals that can propagate (by coupling, radiation or conduction) and interfere with the proper operation of other circuits. To prevent this, interfering and sensitive equipment are shielded from each other, and good ground paths are provided between components of the system. Most designs accomplish this with separate, dedicated low- and high-frequency areas within a card cage, the high-frequency areas including shielded enclosures containing the high-frequency components, i.e., metal enclosures that completely surround the components. The low-frequency area typically contains digital, power, and control circuitry.
As used hereinafter, "high-frequency" refers to signals above 1 MHz and includes signals in the "conventional high frequency" (3 to 30 MHz), very-high frequency (30 to 300 MHz), ultra-high frequency (300 MHz to 3 GHz), and SHF (3 to 30 GHz) bands, including radio frequency and microwave signals. In general, shielding is required for signals above 1 MHz, although it is possible to have some signals above 1 MHz that do not require shielding. As used hereinafter, "nonhigh-frequency" refers to digital, D.C., power, control, and low-frequency (frequencies below 1 MHz) signals or components.